May i ask if you change the food gradually & step by step, or not? Usually, when you change your good brand, to the better quality brand, if you do not change it gradually & step by step, it will likely cause stomach upset, because the better quality food have high protein in it & most likely cats will react to it if they just eat good brand quality before, that has just low protein in it. Also, change the food gradually is really recommended, because cats need time to adjust & change the digestive bacteria system in their stomach to new bacteria who suit to digest their new foods.
If you do change gradually but still have soft stool, more likely you change it too fast. Some cats only need around 1-2weeks to fully adjusted to new food, but some cats need up to 4 weeks to fully adjusted.
For the probiotics, i don't think so. Because probiotics in cats, CMIIW, what i know is they help cats to digest their food (if for humans, they could be use to treat constipation) Because even now I feed my kitten with food that contain probiotics, i still feed him a teaspoon of yogurt (or sometimes, Yakult), twice a day & same as P3 said, the stool is smelly (well, all stools!
) but the smell is drastically decrease the time I switch his food. Because when he arrived last December, he's just been fed Friski*s, & the stool is hard, very dark in color & very smelly. While now, his stool is softer, not so dark (good indication because if you feed them raw/cooked meat, the stool color will be brownish light, so this means the food is good for him), and smells great (great if compare to before!
) & yes, the stool volume is less than before.
For the fruits or veggies i think they just mark up the small amount of the kibble. But if my answer up there are all wrong, then you cat must likely reacted to some ingredient in the food. Maybe the fruits, but still don't know because some cats even allergic to venison, or lamb, or beef. Some cats also react to peas, which been used in many holistic brand catfood nowadays as the main source of 'binding agent'.. So, you can't blame directly to the fruits & veggies as the allergens..
For the clay, yes i have also heard of it. That is why i decided to rotate brands in every 3-4months. Not just to reduce the risk & varied his food, I also don't want him to be finicky & depended/like only on 1 brand all of his life.. & i also mix his food because every brand has their plus & minus point. So, i just think about how to do to maximize the food effect to my kitty
& last thing, how holistic food could be, they still need 'plants' as binding agent. But the choice is up to us as owners to choose which binding agent would you want to feed your cats.. Peas or corn gluten meal. Potatoes or brewers dried rice. Tapioca or whole grain corn..
Hope it helps!